Figure 5. Active site of the chymotrypsin, an enzyme that breaks specific peptide bonds of protein. A phenylalanine residue (blue ring) of a blue protein (could be any protein) slips into the active site of chymotrypsin (green) at its binding site. This positions the peptide bond (red) next to phenylalanine in such a way that it becomes susceptible to cleavage. It becomes susceptible because of a particular confluence of amino acids at the active site. In brief, the oxygen of a serine at the 195th position in the amino acid chain of chymotrypsin is close to the carboxylate group of the phenylalanine residue and can attack the carbon. The carbon is poised for attack because hydrogens from both serine-195 and glycine 193 hydrogen bond with an oxygen from the carboxylate group, reducing the electron density around the carbon. At the same time, the hydrogen that is normally on the serine oxygen is drawn off by the electrons on the ring of histidine at position 57. Those electrons are more available owing to the interaction of a hydrogen on the same histidine ring with the carboxylate group of aspartate at position 102. After the peptide bond is cleaved, the two resulting fragments of the substrate protein float away, leaving the active site of chymotrypsin available for a new substrate.